1. Field of the Invention
The separation of natural oils and other oleaginous mixtures into high purity fractions of their fatty acid constituents is frequently required for investigative and commercial purposes. While the structural diversity of the fatty acids from a given natural triglyceride is somewhat limited, the problem of cleanly separating the acids from one another is often difficult. Moreover, catalytic hydrogenation as practiced in the production of much of our edible oils creates a complex array of positional and geometric isomers, thereby further increasing the difficulty of fractionation. To facilitate handling and to inhibit undesired reactions, the fatty compounds of such oleaginous mixtures are conventionally separated as one of their monobasic, lower alkyl esters. These are prepared by esterification of the free fatty acids or by direct transesterification of the triglycerides. Moreover, it is often the esterified form which is desired as the end product. While there are a variety of fractionating techniques available, most are either limited to a few special types of mixtures or are applicable only to separations on an analytical scale. This invention relates to a versatile method and means for separating substantial quantities of a large variety of fatty ester mixtures into pure fractions.
2. Description of the Prior Art
The use of argentation column chromatography for fractionating fatty esters was first reported by Wurster et al. [JAOCS 40: 513-514 (1963)]. A silver-saturated cation exchange resin was used to separate a mixture of methyl oleate, methyl linoleate, and methyl linolenate by eluting each with a different solvent. The plural solvent system was not only complicated but also precluded solvent recirculation, thereby rendering it commercially impractical. Later, Emken et al. [JAOCS 41: 388-390 (1964)] used a silver-saturated macroreticular cation exchange resin and a single-solvent eluant to obtain relatively pure fractions of both saturated and cis and trans monounsaturated compounds from an esterified and slightly modified olive oil. Some success was also shown for the separation of certain positional isomers of the diunsaturated esters. However, the nonconjugated cis,cis dienes and the cis,cis,cis triene were not eluted. Emken et al. [JAOCS 44(7): 373-375 (1967)] demonstrated that the same argentation technique could be applied to separate conjugated methyl octadecadienoates into cis,cis-, cis,trans-, and trans,trans-isomers. Subsequently, it was taught by Scholfield et al. [JAOCS 54(8): 319-321 (1977)] that with resins having a greater surface area, nonconjugated methyl octadecadienoate can be recovered from an argentated column, but only after extensive elution. Similarly, Emken et al. [JAOCS 55(7): 561-563 (1978)] teaches that with a high surface area resin, mixtures containing saturates, as well as mono-, di-, and triunsaturates, can be readily resolved, and even positional isomers of octadecadienoate can be partially resolved. However, the triunsaturates are not recovered from the column with the eluant and a large volume of solvent is required to elute the diunsaturates. Certain mono- and dihydroxy monoenoic esters have also been separated by silver-saturated resin chromatography [Rakoff et al., JAOCS 55(7): 564-566 (1978)].